How to model spread book interior?
If you are trying to achieve a more organic render you could use an image of a book end or stack of paper as a texture and bump map. The "paper ends" image I captured for the texture/bumpMap is at the bottom of this post. I'm throwing it out to the Public Domaine CC-0.





The body of this book is a cube with some loop cuts which has a Lattice Modifier creating the bend. Then mirrored. 



The two top pages are Bezier Curves which are Extruded. The Resolution of the curves was turned up to smooth the resulting curve.